This invention relates to magnetizing magnets for use with high temperature superconductors (HTSCs).
HTSCs have a critical (transition) temperature, above which they cease to be superconducting, of less than 100xc2x0 K and in many cases below the 77xc2x0 K boiling point of nitrogen. Some of these HTSCs are magnetizable and behave like permanent magnets below their critical temperatures. One such is melt-processed single crystal yttrium barium copper oxide, Yxe2x80x94Baxe2x80x94Cuxe2x80x94O (M-P YBCO), typically having the composition (Y Ba2 Cu3 O7xe2x88x92x).
Use of M-P YBCO involves a number of problems, including the need to be able to generate the fields needed to magnetize it, and the fact that if, for any reason, the material warms up above its critical temperature it will lose all its magnetization. Then, after re-cooling, it requires further magnetization.
The problem of magnetizing the material is that it must be cooled to below 90xc2x0 K (the critical temperature of YBCO) and ideally to below 50xc2x0 K (as the flux it traps increases markedly as its temperature is reduced). In practice, a block of material needs to be held inside a vacuum container as primary insulation supported on insulating struts with sufficiently low conductivity to minimize heat conduction to the block, but with a necessary material strength to support the forces its enormous potential magnetization could cause to act on it in the presence of an external magnetic field. A means of keeping the material cold, typically using a good thermal conductor in contact with it which can be cooled through an appropriate low thermal resistance connection to the cold head of a refrigerator, is also needed and requires insulation. Such insulation and support tends to involve several centimeters of space (perhaps 2 to 4 centimeters) around the block of material.
Such a magnet could be used to provide, or to supplement, the main magnetic field of a magnetic resonance imaging magnet. While such a magnet would have advantages in terms of the strength of field generated in terms of its size, it would suffer the disadvantage of the problems which would be caused if its refrigerator failed and it warmed up to above its critical temperature, which could take from a few minutes to a couple of hours.
In particular, it would be desirable for the magnetizing magnet needed to re-magnetize it to be transportable so that it could be brought to the relevant imaging apparatus, perhaps one of several in a hospital, in order to allow re-magnetization to take place.
However, the bulk of the HTSC in its cryostat, allied to the potentially very high fields needed (up to 8 to 10 Tesla (T)), means that the magnetizing magnet would have to be very large and have a potentially significant fields spread. This would make it very difficult to locate and could make it impracticable to be transportable for re-magnetization of devices in-situ.
According to one aspect of the present invention, a magnet for magnetizing a high temperature superconductor is provided. The magnet includes a pair of poles for generating a magnetizing flux having a cross sectional area and means for imparting relative movement between the area of magnetizing flux and a high temperature superconductor of area greater than the cross-sectional area of the magnetizing flux.
According to a more limited aspect, the magnet includes a cryostat having an evacuated region for containing the high temperature superconductor. The cryostat is moveable between the poles of the magnetizing magnet.
According to another more limited aspect of the present invention, the magnet includes a cryostat having an evacuated region. The poles are disposed within the cryostat. According to yet another more limited aspect, the magnet includes means for cooling successive portions of the high temperature superconductor as the portions pass through the area of magnetizing flux.
According to yet another more limited aspect, the magnet includes a thermally conducting plate and a thermal conductor for thermal contact with a refrigerator. The high temperature superconductor is mounted on the plate, and the conductor is in thermal contact with the plate. According to more limited aspect, a thermocouple is in thermal communication with the high temperature superconductor.
According to another aspect of the present invention, a cryogenic magnetizing apparatus for use with a high temperature superconductor contained in a first evacuated region of a first cryostat is provided. The apparatus includes a second cryostat having a second evacuated region for receiving the high temperature superconductor for magnetization, magnetic field generating windings for generating a magnetizing magnetic field in the second evacuated region, and means for interconnecting the first and second cryostats such that the high temperature superconductor may be transferred between the first and second evacuated regions without substantial loss of vacuum.
According to a more limited aspect, the apparatus includes means for cooling the high temperature superconductor below its critical temperature in the presence of the magnetizing magnetic field. According to another more limited aspect, The apparatus includes means for imparting relative movement between the high temperature superconductor and the magnetizing magnetic field. According to a yet more limited aspect, successive portions of the high temperature superconductor are cooled as they pass through the magnetizing magnetic field.
According to yet another more limited aspect of the present invention, the interconnecting means is an evacuable chamber. The apparatus also includes closures for the first and second evacuated regions. The first and second closures are removable when the chamber has been evacuated.
The high temperature superconductor may include a plurality of high temperature superconductor portions. A carrier having non-magnetizable guides supports the superconductor.
One advantage of an embodiment of the present invention is that a magnetizing magnet with a relatively small field may be used to magnetize a relatively large HTSC.
Yet another advantage of an embodiment of the present invention is that the magnetizing magnet may accommodate the HTSC without its own cryostat during magnetization, enabling the magnetizing magnet to be made smaller and easier to be transported, while allowing return of the HTSC to its own cryostat after magnetization without substantial loss of vacuum.
Still other features and benefits of the present invention will be appreciated by those skilled in the art upon reading and understanding the attached description.